Light that has an optical intensity peak within in a range from the range of visible rays to that of near infrared rays has a high transparency to living bodies. This light can be used to measure data in a living body without attacking the body. This is based on Lambert-Beer's law which teaches that the logarithm value of a measured signal of light is in proportion to the path length of the light and the concentration thereof. This law has been developed. Additionally, a technique for measuring a signal representing a relative change in the concentration of hemoglobin (Hb) (hereinafter “an Hb signal”) in a living body has been developed. The Hb signal that can be measured by the present technique is generally classified into the following three species: an oxidized Hb (“oxy-Hb”) signal, a deoxidized Hb (“deoxy-Hb”) signal and a total signal of oxy-Hb and deoxy-Hb (“total-Hb signal”), which are called an “oxy-Hb signal (species)”, a “deoxy-Hb signal (species)” and a “total-Hb signal (species)”, respectively.
Maki A et al., (1995) “Spatial and temporal analysis of human motor activity using noninvasive NIR topography”, Medical Physics 22, 1997-2005) suggests a technique for measuring Hb signal values of multi-points in the cerebral cortex of a human being simultaneously without attacking the cortex, and this technique has been spreading in research and clinical medicine. The document discloses a method of measuring Hb signal values of the cerebral cortex, thereby measuring brain functions of a human being. Specifically, as the perception mechanism or motor function of a human being is active, the blood flow in the field taking charge of the function inside the cerebral cortex increases. Thus, an oxy-Hb signal species or deoxy-Hb signal species in the field changes. Accordingly, the activity situation of the brain can be evaluated. Typical examples of the change accompanying the brain activity are an increase in an oxy-Hb signal species and a decrease in a deoxy-Hb signal species. These changes are caused by an increase in the blood flow to supplement oxygen and glucose, which are consumed for metabolic activity accompanying neural activity. The increasing blood is arterial and contains oxygen. The increase is far more, as compared with oxygen consumption. It appears that this results in an increase in the oxy-Hb signal species and a decrease in the deoxy-Hb signal species. In general, the increase in an oxy-Hb signal species is more than the decrease in a deoxy-Hb signal species. Therefore, the total-Hb signal species, which is the total of, the oxy-Hb signal species and the deoxy-Hb signal species, increases. It is known that these blood flow changes are generally delayed by about 5 to 7 seconds from neural activity. This technique is beneficial because it allows a subject's brain function to be measured without attacking the subject's brain nor excessively restricting the subject. One beneficial use of this technique is that it can realize the measurement of brain functions of healthy newborns or infants, which has not yet been realized hitherto. Pena M et al. (2003) “Sound and silence: an optical topography study of language recognition at birth”, Proc Natl Acad Sci USA 100(20), 11702-11705; and Taga, G et al. (2003), “Brain imaging in awake infants by near-infrared optical topography”, Proc Natl Acad Sci USA 100(19), 10722-10727).
According to the present technique, three species of an Hb signal, that is, an oxy-Hb signal species, a deoxy-Hb signal species, and a total-Hb signal species can be measured. However, in conventional research on brain functions, there are hardly examples wherein these plural signal species are effectively used. For example, in Maki, et al., which teaches measuring the activity of an adult's motor area, describes a change in the three species of an Hb signal with the passage of time; however, without referring to a difference therebetween substantially, attention is paid to only an increase in the total Hb signal species and his/her brain activity is evaluated. In the same manner, in Pena et al., attention is paid to only an increase in a total Hb signal species and brain activity is evaluated. On the other hand, Taga et al. indicates both of an oxy-Hb signal species and a deoxy-Hb signal species. However, no useful information is extracted from a difference therebetween. For example, from a difference between activity waveforms, it is stated that a decrease in the deoxy-Hb signal species is slower and smaller than an increase in the corresponding oxy-Hb signal species. However, analysis based on the attention of the difference is not made. In past research, including Taga et al., consistency is lacking in the results for a deoxy-Hb signal species than in those of the corresponding oxy-Hb signal species, the activity of which is clearly shown; therefore, about the deoxy-Hb signal species, only a matter that the signal-to-noise ratio thereof is low is studied in many cases. This fact is based on a presupposition that an oxy-Hb signal species and the corresponding deoxy-Hb signal species originally indicate the same brain activity. As described above, in spite of the existence of the technique capable of measuring the three species of an Hb signal, there has not been any method of using a difference or a common facture therebetween to improve the detection precision of brain activity.